Pressure dependence of the ideal tensile strength of bcc niobium*

The pressure dependence of the ideal tensile strength of bcc niobium was investigated using the {\it ab initio} pseudopotential density-functional method. The external stress is applied perpendicular to the main tensile strain direction. With positive external stress, the stresses of both the tetragonal Bain path and the orthorhombic path move up to higher values, resulting in higher ideal tensile strength. Also, the branching of these two paths moves out to higher strain. With negative (compressive) external stress, the ideal tensile strength decreases, and the branching of the orthorhombic path and tetragonal path moves to smaller strain. The change in the ideal tensile strength can be explained using simple arguments.